Let us call a red-black tree strict when every black node has at most one red child.
Show that a strict red-black full tree has at most $(n − 1)/4$ red nodes; a binary tree is full when every node has zero or two children.
The hint for this problem says to use a charging argument but I can't seem to figure one out.
My idea was to consider the root of the tree (which must be black) and recursively consider the number of red nodes in the left and right subtrees and show by induction that this holds. But I can't seem to figure out some of the details (and there seem to be some inconsistencies).